Question

My test RegExp with pattern before returns null. How do I get it to return 2?

s = new RegExp(/(?=ID\=)(\d+)/).exec("ID=2");

Answer 1

(?=) is a lookahead assertion. You’d want to use lookbehind, which JavaScript does not support. This, however, will work just fine:

s = /ID=(\d+)/.exec('ID=2')[1];
// or
s = new RegExp('ID=(\\d+)').exec('ID=2')[1];

Don’t pass a regexp literal to the RegExp constructor — that just does not make sense.

Really, though, why wouldn’t you just split on =?

s = 'ID=2'.split('=')[1];